Worldwide demand for propylene is growing continuously and in recent years propylene prices have exceeded that of ethylene. Almost 60% of the total propylene is produced by steam cracking of various hydrocarbon streams like Naphtha, Gas oil and Liquid Petroleum Gas (LPG). One of the cheapest way to obtain propylene is from Fluid Catalytic Cracking (FCC), which contributes to >30% of the total propylene production.
Zeolite is one of the most widely used catalytic materials in hydrocarbon conversions. It is being widely used as catalyst and/or additive in catalytic crackers or incorporated in cracking catalysts. The use of cracking catalyst comprised of a large pore size crystalline zeolite (pore size greater than 7 angstrom units) in admixture with ZSM-5 type zeolite for improving the octane number has been reported in U.S. Pat. No. 3,758,403. When a conventional catalyst containing 10 percent REY is added with ZSM-5 molecular sieve in the range of 1.5 percent to 10 percent, the gasoline octane number and the yield of lower olefins are increased. However, it has been found that the increasing amplitude is reduced with increasing amount of ZSM-5 molecular sieve. Using an additive that contains ZSM-5 molecular sieve has the same effect.
Similar combinations of ZSM 5 with a zeolite cracking catalyst of the X or Y faujasite variety have been described in U.S. Pat. Nos. 3,894,931; 3,894,933; and 3,894,934.
Researchers have attempted to take advantage of the cracking activity/selectivity of ZSM-5 in different proportions. These have been reported in numerous patents such as U.S. Pat. Nos. 4,309,279 and 4,309,280.
Use of pre-treated zeolite, particularly ZSM-5 in the additive catalyst in combination with FCC catalyst has been widely reported. For example, use of thermally treated zeolite for its use in FCC has been reported in U.S. Pat. No. 4,552,648.
Apart from its activity and selectivity one of the most desirable attributes of the FCC catalyst and additive is its hydrothermal stability. The regeneration conditions in a FCC unit are quite severe (typically 690-800° C. in the presence of steam) and the additive and the catalyst, specifically zeolites are very much susceptible.
Under these conditions de-alumination of the zeolite takes place, resulting in a loss of Al—OH—Si groups responsible by the Brönsted acidity. Preventing or minimizing de-alumination is a topic of continuous interest in the field of FCC applications. Exchange of rare earth (RE) retards destruction of the Y zeolite during the hydrothermal treatment which also results in an increase in the strength of acid sites enhanced cracking activity. However, increase in RE, promotes hydrogen transfer activity and thereby reduces the propylene yield. Hence, in order to maintain the activity and also to minimize hydrogen transfer, optimum amount of RE is exchanged and higher amount of U.S.Y zeolite is used.
One of the known approaches for improving the hydrothermal stability of the ZSM-5 additives is treatment with Phosphates. In the case of ZSM-5 zeolite, phosphorus compounds interact with bridged OH groups, thereby decreasing the zeolite acidity and affecting the catalytic activity. Blasco et al. (J. Catal. 237 (2006) 267-277) describes different proposed models by several researchers for surface structure of phosphate in ZSM-5 zeolite. The acidity reduction by framework dealumination and formation of aluminum phosphate has been reported. Thermal treatment of H3PO4 impregnated HZSM-5 causes less dealumination than that of the same treatment of un-impregnated HZSM-5 indicating that phosphorous partially protects Al from being removed from the framework. This is well known in the prior art. Considerable work has been done by formulating and optimizing catalyst/additive compositions.
Generally the FCC catalysts/additives are deactivated at above 750° C. in the laboratory/pilot plant to simulate commercial FCC plant yields. Close predictions have been observed only for FCC catalyst and on the contrary, ZSM-5 containing additives are less active in commercial plants than the laboratory predictions for LPG and propylene yield.
FCC Cracking catalyst containing phosphate treated zeolites is disclosed in U.S. Pat. No. 5,110,776. According to the process USY/REY zeolite is contacted with a phosphate salt prior to clay-sodium silicate-sulfuric acid addition. In the catalyst disclosed in U.S. Pat. No. 5,110,776, sodium silicate is the major binder. It has been reported that phosphate treatment of the aluminum oxide containing matrix material leads to the formation aluminum phosphate which acts as a glue in the matrix and this leads to the improvement in the attrition resistance.
Various FCC processes that employ phosphorous treated zeolite, especially ZSM either as FCC catalyst or as an additive has been reported in U.S. Pat. Nos. 5,231,064 , 5,348,643, 5,472,594, 6,080,303, U.S. Pat. 2003/0047487, U.S. Pat. Nos. 5,472,594, 5,456,821, 6,566,293 and WO 98/41595.
Numerous studies on the performance of ZSM-5 additive have been reviewed by Degnan et al. (Microporous and Mesoporous Materials 35-36 (2000) 245). Demmel et al. (U.S. Pat. No. 5,190,902) teaches the preparation methods for attrition resistant binders wherein a slurry of clay particles is brought to either a low pH level (1 to 3) or to a high pH level (10 to 14) and is mixed with a phosphorous containing compound in a concentration of 2 to 20 wt %.
Also U.S. Pat. No. 5,231,064 discloses the preparation and use of ZSM containing catalytic cracking catalysts containing phosphorous treated clay prepared at pH less than 3. Further, U.S. Pat. No. 5,126,298 also discloses the preparation of additive having attrition resistance in the range of 5-20. According to the claims, pH of final catalyst slurry prior to spray drying is less than 3.
U.S. Pat. No. 6,858,556 teaches the preparation of stabilized dual zeolite in a single particle catalyst composition consisting of 5% ZSM-5 and 12% REY using conventional silica-alumina binder for cracking of heavier hydrocarbons into lighter products.
U.S. Pat. Nos. 7,585,804, 7,547,813, 7,375,048, and 5,521,133 disclose attrition resistant FCC additive containing at least 30% ZSM-5. The phosphoric acid is injected into the mixture of highly dispersed kaolin slurry, ZSM zeolite, reactive and non-reactive alumina to make attrition resistant additives by employing on-line mixing of phosphoric acid with zeolite-alumina-clay slurry to minimize contact time and avoid viscosity.
Ziebarth et al. (U.S. Pat. No. 6,916,757) discloses the preparation of FCC additive at pH below 3, containing ZSM-5 zeolite, phosphate and alumina. The alumina content has been optimized to have Attrition Index (AI) of about 20 or less for an additive containing zeolite content of 30-60 wt %. The additives are deactivated at 815° C. (1500 F) for 4 hours prior to Micro Activity Test (MAT).
A hydrothermally stable porous molecular sieve catalyst and a preparation method thereof is disclosed in Choi et al. (U.S. Pat. No. 7,488,700). The method disclosed by Choi et al comprises the steps of adding molecular sieve to aqueous slurry phosphate and water soluble metal salt and finally removing the water by evaporation process. Its been reported that the catalyst maintains its physical and chemical stabilities even after hydrothermal deactivation in an atmosphere of 100% steam at 750° C. for 24 hours. The catalytic properties are evaluated in a fixed bed naphtha cracking and catalysts are found to be active and more selective to olefins (C2 and C3).
U.S. Pat. No. 7,601,663 discloses the preparation of solid acid catalyst and producing light olefins from hydrocarbon stocks mainly for naphtha cracking. The method disclosed involves the use of pillaring binding agent, which is prepared by reaction of aluminum salt with phosphorous compounds.
A Process for catalysts component or additives, more resistant to the hydrothermal deactivation, employed in fluid catalytic cracking processes is disclosed in Lau et al. (U.S. Pat. No. 2007/0173399). The process involves the use of a low Na2O content zeolite which is treated with phosphorous in the presence of water vapour. The phosphorous content deposited as P2O5 ranges between 1% and 10% w/w in relation to the weight of the zeolite. The hydrothermal deactivation studies are carried out at 800° C. for 5 hours.
Most of the commercial FCC units, use more than 9-10% ZSM-5 crystals to maximize propylene yields. Also refiners look for hydrothermally stable ZSM-5 additive to increase the propylene yield and also to sustain for a longer period.
U.S. Pat. No. 7,517,827 discloses a process for preparing a catalyst composition for cracking heavy hydrocarbon which employs a high silica low soda medium pore zeolite. In accordance with process provided in U.S. Pat. No. 7,517,827, the clay slurry is treated with a phosphate source independently and zeolite slurry is treated with ammonical solution. The combination of treated zeolite, the alumina binder, and the phosphate-clay slurry is spray dried and calcined. The precursor slurry pH of 1-3 prior to spray drying improves the attrition resistance.
FCC catalyst/additives with mere high selectivity and high conversion rate are very much desirable but these attributes in themselves are not sufficient to make the overall cracking process efficient and economical. Though it has been possible to attain high propylene yield using the additives hitherto reported, sustaining it over a period of time still remains a challenge.
Cao et al. (U.S. Pat. No. 6,080,303) discloses a process which comprises the steps of treating a zeolite with a phosphorus compound to form a phosphorus treated zeolite and combining the phosphorus treated zeolite with AlPO4. The catalyst composition as taught in Cao et al comprises 0.5 to 10 wt % phosphorous, 1-50 wt % AlPO4, 5-60 wt % zeolite and a binder material.
Kowalski et al. (U.S. Pat. No. 5,318,696) discloses a catalytic cracking process which employs a catalyst composition comprising a large-pore molecular sieve material having pore openings greater than about 7 Angstroms and an additive catalyst composition comprising crystalline material having the structure of ZSM-5 and a silica/alumina mole ratio of less than about 30. The additive catalyst is prepared by a) synthesizing ZSM-5 crystals, b) slurring ZSM-5 with matrix material such as silica, alumina, silica-alumina or clay and if desired phosphorus to make ZSM-5/matrix composition at a pH of 4-6 and spray drying, c) converting the dried ZSM-5 matrix composition to protonic form by acid treatment (e.g., 0.1 to 1 N HCl)/ammonia exchange and/or calcination. The method essentially necessitates the method step of washing for removing sodium sulphate and soda of the ZSM-5 zeolite which are used for preparing a silica-alumina binder.
Demmel et al. (U.S. Pat. No. 5,958,818) discloses a process for preparation of clay/phosphate/zeolite catalyst using clay phosphate as binder by age-reaction of clay phosphate/clay-zeolite-phosphate up to 24 hrs in the pH range of 7 to 14. The proportion of clay in the catalyst prepared by the method provided in U.S. Pat. No. 5,958,818 between 50 to 94.5 wt % with respect to
It is well known that it would be difficult to bind zeolite with only clay phosphate system to obtain desired attrition properties even for a low zeolite content (<20%) for FCC formulations. Further, the said patent claims that optimization of beta with total zeolite content of 12 wt % in the above formulation, has shown an improvement in gasoline octane and propylene yield. Though, the hydrothermal deactivations were carried out at 760° C. for 5 hrs, which are mild conditions to predict the stability of additives in commercial FCC plant.
The currently available commercial ZSM-5 additives, having 25-50 wt % zeolite crystals, do not sustain propylene yield in the commercial plant due to continuous deactivation of ZSM-5 and hence, there is a need for a process to provide hydrothermally stable FCC catalyst additive with attrition resistance. The present invention addresses the issue of sustainable propylene yield even after severe hydrothermal deativations for durations more than 100 hours.
In the present invention, the phosphates are effectively used to stabilize the zeolite by ageing and also to minimize clay-phosphate interaction during preparation and further discloses the synergic effect of silica/silica-alumina (binders) with zeolite-phosphate stabilization led to high stability and desired attrition properties.
Definitions
As used in the present specification, the following words and phrases are generally intended to have the meanings as set forth below, except to the extent that the context in which they are used indicate otherwise.
Phosphorous stabilization means effective interactions of ZSM-5 zeolite and phosphate to minimize/prevent the dealumination of zeolite during hydrothermal deactivations under FCC conditions.
Normal hydrothermal deactivation conditions correspond to deactivation at 800° C. with 100% steam for ≦20 hrs.
Severe hydrothermal deactivation conditions correspond to deactivation at 800° C. with 100% steam for ≧20 hrs.
Objects of the Present Invention
An object of the present invention is to provide a process for preparation of ZSM-5 additive for maximization of lower olefin yields (C2-C4 hydrocarbons) primarily propylene yield in FCC.
Another object of the present invention is to provide a process for preparation of a FCC catalyst additive that is capable of sustaining propylene yield for a time period of at least 100 hours.
Yet another object of the present invention is to provide a process for preparation of a FCC catalyst additive which is substantially devoid of sodium.
Still another object of the present invention is to provide a steaming protocol for ZSM-5 additive deactivation for close prediction of plant yields.